There are many different sizes your quilt could be. The main difference would be the depth of your mattress itself.
Say a standard queen size mattress is 60x80. No.w you need to add the depth of your mattress to both sides (my mattress is 15"), so I have to add 30+60= 90 and then say add on how much below the mattress line I want it to hang. Say 3", so now I also have to add 6"+ 90"= 96" for a total width of 96". Anything smaller would not fit my beds width.
Then you need to remember to do the same for the length. So I already know my mattress is 80" so 80"+15"=95". Now that's fine if it goes flat on the bed with say pillow shams on top of it, but it you want it to go over your pillows you need to add say another 6"-8" (I'll use 6" here) to allow for the poof of the pillows, so now we're up to 101". Do you want to tuck it under your pillows add an additional amount of say 6-8". So now we're up to 107-109".
I guess what I'm trying to say is one size doesn't fit all. :-D